1. Field of the Invention
The present invention relates to an aluminum heat exchanger having excellent corrosion resistance. More particularly, the present invention relates to an aluminum heat exchanger in which the corrosion resistance of a tube material is improved in an automotive heat exchanger which is assembled by brazing an aluminum fin material to the outer surface of a tube material made of aluminum (including an aluminum alloy) formed by bending a sheet material.
2. Description of Background Art
An automotive aluminum heat exchanger, such as a condenser or an evaporator, is generally manufactured by brazing a tube in which a refrigerant flows and a fin which exchanges heat with the outside. It is important to protect the outer surface of the tube material against corrosion in order to secure corrosion resistance of the heat exchanger. Conventionally, the outer surface of the tube material is protected against corrosion by using a method of utilizing sacrificial corrosion of the fin material or a method of forming a Zn diffusion layer on the surface of the tube material.
However, in the case where low chlorine ion water serves as a corrosion environment, such as an evaporator, since it is difficult to obtain a potential necessary for corrosion protection in the area apart from the joint section between the tube and the fin by using the method of utilizing sacrificial corrosion of the fin material, sufficient corrosion protection of the tube cannot be achieved.
In the method of forming a Zn diffusion layer on the surface of the tube material to protect the tube material against corrosion by utilizing the sacrificial corrosion effect of the outer surface of the tube, the Zn diffusion layer is formed on the surface of the tube material by Zn thermal spraying when an extruded tube is used as the tube material, and a fin formed of a brazing sheet on which an Al—Si alloy filler metal is clad is brazed to the tube. In the case where a tube formed by bending a sheet material is used as the tube material, a tube material formed by bending a sheet material on which an Al—Si alloy filler metal containing Zn is clad on the surface is brazed to a bare fin on which a filler metal is not clad (see Japanese Patent Application Laid-open No. 2001-71172). It is advantageous to use the bare fin instead of a fin formed of a brazing sheet from the viewpoint of surface treatment capability, thermal conductivity, and brazeability.
In recent years, a reduction of the thickness of the heat exchanger material has been strongly demanded accompanying a demand for a reduction of the weight of the heat exchanger due to a reduction of the weight of vehicles. From this viewpoint, it is difficult to reduce the thickness to a large extent by the method of using an extruded tube as the tube material. The thickness can be reduced by using a tube formed by bending a sheet material as the tube material. However, sufficient corrosion resistance cannot necessarily be secured since the Zn diffusion layer is rapidly consumed.
The outline of corrosion of an aluminum heat exchanger formed by brazing a tube material to a fin material is described below. As shown in FIG. 1, a bare fin 1 formed of an Al—Mn alloy is combined with a tube material 2 formed by bending a sheet material in which an Al—Si alloy filler metal containing Zn is clad on an aluminum alloy core material 4. When the bare fin 1 and the tube material 2 are heated for brazing, a Zn diffusion layer 3 is formed on the surface of the tube material 2, and the filler metal 3 is melted to form a fillet F, whereby the bare fin 1 and the tube material 2 are brazed.
The potential of the surface of the tube material 2 must be lower than the potential of the core material 4 of the tube material 2 from the viewpoint of corrosion protection of the tube material 2. In order to decrease the potential of the surface of the tube material, Zn is added to the Al—Si alloy filler metal 3, and the Zn diffusion layer 3 is formed on the surface of the tube material 2 during heating for brazing. However, since consumption of the Zn diffusion layer in a normal corrosive solution is increased due to Si diffused together with Zn, penetration corrosion tends to occur directly under or near the brazed section in the early stages.
A method which aims at solving the above problem instead of the method of forming the Zn diffusion layer by using a tube material formed by bending a sheet material in which an Al—Zn alloy is clad on the outer surface of a core material formed of an Al—Mn alloy equivalent to A3003 or A3103 as the tube material, and forming a sacrificial corrosion layer with a small corrosion rate by brazing an aluminum fin material to the Al—Zn alloy layer formed on the outer surface of the tube material has been proposed (see Japanese Patent Application Laid-open No. 2001-50690). However, this method does not necessarily provide sufficient corrosion resistance depending on the use environment of the automotive aluminum heat exchanger.